Slackware's initrd.img changed from gzip encryption to lz. 
The old method of 
```
zcat /boot/initrd-$(uname -r).img | cpio -idmv
```
to extract slackware's boot initrd, has become:
```
xz -dc < /boot/initrd-$(uname -r).img | cpio -idmv
```

One of the reasons I need to know how to extract a Slackware initrd is
to chroot into it. I chroot into the Slackware initrd and use it to
install Slackware on remote servers. 

Sources: https://access.redhat.com/solutions/24029
